When an automobile brakes on a split road (i.e., a road having different frictional characteristics) a frictional coefficient between the road and the right wheel may be different from the frictional coefficient between the road and the left wheel. As a result, a yaw moment may be generated on the automobile based on the difference in braking forces between the left and right wheels. Under some bad conditions, the automobile will turn suddenly due to the generated yaw moment.
To reduce such yaw moment, Japanese Laid-open Patent Publication No. H03-96465 discloses a "low select" control which reduces the brake pressure for one of the front wheels which has a higher frictional coefficient based on a brake pressure for the other front wheel which has lower frictional coefficient. The "low select" control is performed for a predetermined time period Tk at the time of anti-locking brake control. The "low select" control ends after the time period Tk has elapsed. At that moment, if a speed of the wheel which has a lower frictional coefficient is below a predetermined value, the control judges that the automobile is travelling on the split road. Upon determining that the automobile is travelling on such a road, the control increases the pressure for the wheel which has the higher frictional coefficient.
However, if the time period Tk is short, the yaw moment cannot be reduced sufficiently and thus, the automobile may experience sudden large turning movements. On the other hand, if the time period Tk is long, the automobile cannot be braked sufficiently so that more distance is required to stop the automobile. Thus, the time period Tk has to be properly determined. However, the proper value of the time period Tk may vary depending upon the road conditions and/or the depressing force applied to the brake pedal. Therefore, optimum control cannot be performed if the time period Tk is a fixed value.
Additionally, the brake pressure is increased as soon as the time period Tk is elapsed. Therefore, the yaw rate may be increased once again after the time period Tk has elapsed.